In the Dutch patent application NL 7612518 an installation for manufacturing an extruded plastic section is disclosed wherein the extruded tubular section is cooled not only externally, which is the most common cooling method, but there is also internal cooling with cooling water which comes directly into contact with the inside of the plastic section to be cooled. As a result of said additional internal cooling, cool-down of the section can be improved and, at the same time, the length of the cooling section can be considerably shorter, theoretically by a factor of four, than if external cooling only were to be employed.
In the case of the known installation, for the purpose of internal cooling with cooling water, a sealed compartment is bounded by the insulated end face of the internal mandrel of the extruder on the one hand, and a closing means which is situated at a distance downstream therefrom in the extrusion direction on the other hand. The closing means, in the case of the known installation, is a cylindrical metal stopper which, via a hollow anchor rod, is fixed to the mandrel of the extruder. The external diameter of the stopper is such that the plastic section, which is cooling down and therefore shrinking, slides with friction along the rigid outer wall of the stopper. In the process, the contact pressure between the stopper and the plastic section brings about the fluid seal intended for the purpose of sealing the compartment, this being assisted by a plurality of circumferential grooves in the stopper, which provide the effect of a labyrinth seal. Through the hollow anchor rod cooling water can be introduced, upstream of the closing means, into the sealed compartment, said cooling water then flowing in a counter-current direction along the plastic profile to be cooled and then being discharged via the mandrel of the extruder.
The method and installation disclosed by NL 7612518 has not proved satisfactory in practice, particularly with respect to the closing means employed there. The seal obtained with said known closing means cannot be controlled in practice, as a result of which there will occur either undesirable leakage or precisely the problem of the plastic section lying with such force against the rigid circumferential wall of the stopper that as a result of the large frictional forces then produced damage is caused to the plastic section and the mounting of the stopper is subjected to undesirably large (tensile) loads.
For the abovementioned reasons, the purpose of effecting internal cooling has until now been served by preferably employing flexible closing means, for example as described in DE 25 06 517. In the case of the closing means shown in said publication, the outer wall of the closing means is made of a flexible material, and the pressure with which the flexible outer wall lies against the section to be cooled can be controlled. Since extrusion installations are in practice operated continuously, the inevitable result, inter alia under the influence of the prevailing temperatures and pressures and the frictional contact with the extruded section, is wear of the flexible wall of the closing means, which necessitates regular replacement thereof, for which purpose the extrusion installation has to be shut down.